Exemplary embodiments relate to a nonvolatile memory device and a method of performing a program operation using the same and, more particularly, to a nonvolatile memory device and a method of operating the same, which are capable of preventing an increase in a threshold voltage.
With a gradual increase in the degree of integration of nonvolatile memory devices, a multi-level cell (MLC) capable of storing a number of bits is being used in order to store more data.
FIG. 1 is a cross-sectional view of a nonvolatile memory device for illustrating known concerns. In particular, this figure shows part of the cross section of a memory cell array included in the nonvolatile memory device.
The memory cell array includes a number of strings. Only one of the strings is shown in FIG. 1. The string includes a number of word lines WL0 to WLn, a drain selection line DSL, and a source selection line SSL formed over a semiconductor substrate 10. The word lines WL0 to WLn preferably are formed between the drain selection line DSL and the source selection line SSL. Each of the word lines WL0 to WLn, the drain selection line DSL, and the source selection line SSL is implemented by stacking a gate insulating layer 12, a floating gate 14, a dielectric layer 16, and a control gate 18. In the drain selection line DSL and the source selection line SSL, a hole is formed through a part of the dielectric layer 16, and so the floating gate 14 and the control gate 18 are electrically coupled together. A junction 10a is formed in the semiconductor substrate 10 between the word lines WL0 to WLn, between the drain selection line DSL and the word line WLn, and between the source selection line SSL and the word line WL0. When a driving voltage is supplied to each of the lines, a channel is formed under the lines.
An MLC has a number of threshold voltage ranges. Thus, a difference in the voltage level between a threshold voltage having the lowest level and a threshold voltage having the highest level is increased with an increase in the number of threshold voltage ranges. In particular, with an increase in the level of a threshold voltage, the level of a pass voltage supplied to unselected word lines during a program or read operation also rises. When the level of a pass voltage supplied to the word lines rises, the voltage level of channel boosting generated in the channel regions of unselected strings during a program operation also rises.
Meanwhile, in the case in which there exists a memory cell with a threshold voltage having a high level, channel boosting does not regularly occur, and so the level of channel boosting within the same string may differ. In this case, hot electrons can be generated because an electric field is generated in a region in which the voltage level of channel boosting differs.
FIG. 2 is a graph illustrating an increase in the threshold voltage in a related art.
If hot electrons are introduced into the floating gate of a memory cell, the threshold voltage of the memory cell can shift. For example, if hot electrons are introduced into a memory cell, the threshold voltage can rise from 20 to 22 as shown in FIG. 2. When the threshold voltage rises, the interval between different threshold voltage ranges is narrowed and may deteriorate reliability of the memory cell during a subsequent read operation.